Irish Dentist about precision milling

CLINICAL

Telescopic crowns in practice In the first of two articles, Ulrich Heker and Verena TunnSalihoglu discuss the technical side of telescopic dentures, focusing on the clinical aspects of the treatment in next month’s instalment Telescopic prostheses (also known as double crowns) are a proven option for prosthetic treatment where there is dramatically reduced dentition (and in some cases might even serve as an alternative to implants).

However, the production of such a prosthesis demands considerable technical skills from both the dentist and the dental technician; of equal importance is good communication between the dental team.

The telescopic attachment consists of two parts: 1. The primary (inner) crown, or coping, which is permanently fixed to the anchor teeth and is preferably made out of a precious metal – a high-gold alloy 2. The secondary (outer) crown incorporated into the prosthesis, made out of the same alloy.

Ulrich Heker is a Master Dental Technician and the owner and manager of TEETH‘R’US in Essen, Germany.

Verena Tunn-Salihoglu is a practising dentist in Aberdeen, Scotland. www.IrishDentist.ie

The usual type of double crown system, next to the conus type, is the parallel telescopic crown.

They are so-named because all of the surfaces of the primary (inner) and the secondary (outer) telescope are not only parallel to one another but also parallel to the axis of each incorporated tooth.

However, in the case of a conus telescopic system a 4º angle of both telescopes to the axis of the tooth is aimed for, based upon by the exact preparation carried out by the dentist.

Indications Double crowns can be used in the following situations: • Where there is a strongly depleted dentition with uncertain prognosis of individual teeth in a periodontally damaged jaw (existing bone depletion, increased loosening of the anchor teeth) with a suboptimal distribution of the remaining teeth

• For the retention of removable bridges.

The almost universal applicability is characteristic of this anchoring system.

Telescopic crowns can be applied as clasp-free connecting elements with purely periodontally and periodontally-gingivally supported partial prostheses.

The pros and cons of double crowns The advantages of the telescopic system include: • A predominantly axial loading of the pillars, leading to a favourable distribution of force and thus helping to protect the anchor teeth from decay • The option of primary splinting for securing and fixing loose teeth • Integrated tilt-avoidance • A straightforward ability to extend the prosthesis even up to a full denture, with a distinct aesthetic advantage because no clasps are used.

The treatment allows for control of the periodontium

and the internal coping • A cost-effective alternative to implants.

Disadvantages of the telescopic system include: • It requires a high level of technical expertise • There are correspondingly higher costs • Over-sizing of the secondary crowns can occur if the pile has not been efficiently reduced • Compared to a porcelainfused-to-metal (PFM) crown, higher tooth substance loss during preparation can only be covered/veneered with composites

How double crowns work Physical principles The patient expects the prosthesis to be inserted and removed easily. At the same time, the prosthesis has to be sufficiently attached so that it cannot be leveraged off by motion during speech and eating. For this to be achieved, some existing physical needs must be met.

October 2010 | Irish Dentist | 21

CLINICAL

In order to obtain a hold between the primary and secondary telescopic crown elements, they have to fit in a particular manner. There are three different types of fit: 1. A clearance fit, in which there is still a small bit of give/play 2. A medium fit, in which there is a large tolerance or over-sizing before the joining of the components (which gives totally useless telescopic crowns) 3. The pressure fit, where the components are tight and interact such that friction is created during fitting.

On the basis that both crown pieces have to join exactly and without obstruction, parallel telescopic crowns are always of the pressure-fit type, which is why there is the preference that telescopic crowns are made from precious alloys, given their high elasticity.

The importance of friction The inner and outer telescopes stay together because of friction. Stated simply, the friction is due to the interaction between the surface layers of the inner and outer telescope and the binding forces of the telescopic crowns are a consequence of this friction.

Friction in telescopic crowns is a value that is difficult to measure. It is principally dependent upon the technical construction of the crown, which is influenced by the following factors: • The number of planned telescopic crowns • The length of the friction surfaces of the individual tooth and the sum of all available telescopes • The placement of the friction surfaces relative to one another. Only oppositefacing parallel surfaces can provide the required friction with the elasticity of the materials • The quality of the work.

The prosthesis has to be prepared in such a way that 22 | Irish Dentist | October 2010

the patient can insert it without difficulty.

Additionally, it must provide the feeling of fitting firmly.

The denture should also be removable without difficulty, while not loosening at the wrong moment. All of these criteria must remain valid over a long wear period.

(NB: The force required for removal of the prosthesis – 250-300 P – is regarded as acceptable to patients. The maximum force required for removal should not exceed 650 P, because if a higher level of force is needed, often the patient cannot manage.)

Achieving the correct level of friction for the individual telescopic components is only possible as a result of considerable experience and skill by both parties technically involved – the dentist and the dental technician – and their interdisciplinary communication.

The success also depends on the precision of each step and a keen eye for detail.

A telescopic crown during insertion

Principles and next steps This article provides an overview of the technical aspects of telescopic prostheses or double crowns and their almost universal applicability. The basic principles of how they work and the importance of achieving the right level of friction are described.

Success depends upon good communication and technical skill by both the dentists and the technician.

Look out for the second article on our precision dental prosthetics with highly engineered connections, which will be published in the next issue of Irish Dentist and will illustrate the clinical aspects of this kind of treatment modality.

For a full list of the references used in the writing of this article, please email refs@irishdentist.ie.

A single telescopic attachment with retention, showing the primary and secondary crowns

This upper jaw prosthesis is now ready for modelling of the outer crowns www.IrishDentist.ie